An X-ray CT apparatus acquires data resulting from X-ray transmission through an object, while rotating a pair of an X-ray source and an X-ray detector which are placed facing each other with the object positioned therebetween and reconstructs a tomographic image (hereinafter referred to as a CT image) of the object through calculation and the apparatus is used as, inter alia, inspection equipment for industrial use and security application and diagnostic imaging equipment for medical application.
One type of X-ray CT apparatus for medical application is a PCCT apparatus provided with a photon counting mode. The PCCT apparatus counts photons of X-ray (X-ray photons) that transmitted through an object per element to be detected by a photon counting scheme based X-ray detector, as found in, e.g., PLT 1. Through this it is possible to obtain, for example, a spectrum enabling presumption of elements constituting an internal tissue of the object through which X-ray transmitted and to obtain an X-ray CT image in which element level differences are visually represented in detail.
Also, the PCCT apparatus is able to obtain X-ray intensity per energy value by discriminating counted X-ray photons by energy value. Using this, the PCCT apparatus extracts only X-rays within a particular range of energy and makes imaging of the X-rays, which may be used for diagnosis.
The X-ray CT apparatus images an object from different angles, while rotating the X-ray source; this process involves a fluctuation in results of temporal integration in a rotational direction due to errors in spacing between notches in the rotational direction (this fluctuation will be referred to as a temporal fluctuation in the rotation direction hereinafter). That is, errors occur with intervals, for each of which individual pieces of data in the rotational direction are integrated (each of the intervals will be referred to as a view). Because view length varies, there is a larger number of data in a longer interval and, conversely, there is a smaller number of data in a shorter interval. Moreover, the X-ray intensity of the X-ray source fluctuates over time. Because of these errors, an artifact is generated in an image which is obtained with the X-ray CT apparatus. For preventing such artifact, the X-ray CT apparatus uses a reference detector to reduce such errors. In particular, an X-ray detector for reference is placed in a location where X-ray does not transmit through an object and performs measurement in the same timing as an X-ray detector located to detect X-ray that transmitted through the object; thereby, data is acquired with regard to a temporal fluctuation in the rotational direction due to errors in spacing between notches in the rotational direction and a fluctuation pertaining to the X-ray source. Then, corrections are made with a ratio between the outputs of the X-ray detector and the reference detector with respect to each view; thus, these two kinds of errors can be corrected.